Semiconductor devices often include multiple layers of conductive, insulating, and semiconductive layers. Often, the desirable properties of such layers improve with the crystallinity of the layer. For example, the electron mobility and electron lifetime of semiconductive layers improve as the crystallinity of the layer increases. Similarly, the free electron concentration of conductive layers and the electron charge displacement and electron energy recoverability of insulative or dielectric films improve as the crystallinity of these layers increases.
For many years, attempts have been made to grow various monolithic thin films on a foreign substrate such as silicon (Si). To achieve optimal characteristics of the various monolithic layers, however, a monocrystalline film of high crystalline quality is desired. Attempts have been made, for example, to grow various monocrystalline layers on a substrate such as germanium, silicon, and various insulators. These attempts have generally been unsuccessful because lattice mismatches between the host crystal and the grown crystal have caused the resulting layer of monocrystalline material to be of low crystalline quality.
If a large area thin film of high quality monocrystalline material were available at low cost, a variety of semiconductor devices could advantageously be fabricated in or using that film at a low cost compared to the cost of fabricating such devices beginning with a bulk wafer of semiconductor material or in an epitaxial film of such material on a bulk wafer of semiconductor material. In addition, if a thin film of high quality monocrystalline material could be realized beginning with a bulk wafer such as a silicon wafer, an integrated device structure could be achieved that took advantage of the best properties of both the silicon and the high quality monocrystalline material.
Epitaxial growth of monocrystalline oxide thin films on silicon has numerous potential device applications, such as, for example, ferroelectric devices, high density memory devices, and next-generation MOS devices. Some of these oxides, such as BaO and BaTiO3, were formed on silicon (100) using a BaSi2 (cubic) template by depositing one fourth monolayer of Ba on silicon (100) using molecular beam epitaxy at temperatures greater than 850° C. See, e.g., R. McKee et al., Appl. Phys. Lett. 59(7), p. 782–784 (12 Aug. 1991); R. McKee et al., Appl. Phys. Lett. 63(20), p. 2818–2820 (15 Nov. 1993); R. McKee et al., Mat. Res. Soc. Symp. Proc., Vol. 21, p. 131–135 (1991); U.S. Pat. No. 5,225,031, issued Jul. 6, 1993, entitled “PROCESS FOR DEPOSITING AN OXIDE EPITAXIALLY ONTO A SILICON SUBSTRATE AND STRUCTURES PREPARED WITH THE PROCESS”; and U.S. Pat. No. 5,482,003, issued Jan. 9, 1996, entitled “PROCESS FOR DEPOSITING EPITAXIAL ALKALINE EARTH OXIDE ONTO A SUBSTRATE AND STRUCTURES PREPARED WITH THE PROCESS.” A strontium silicide (SrSi2) interface model with a c(4×2) structure was proposed. See, e.g., R. McKee et al., Phys. Rev. Lett. 81(14), 3014 (5 Oct. 1998).
Growth of SrTiO3 on silicon (100) using an SrO buffer layer has been accomplished. See, e.g., T. Tambo et al., Jpn. J. Appl. Phys., Vol. 37, p. 4454–4459 (1998). However, the SrO buffer layer was thick (100 Å), and crystallinity was not maintained throughout the growth. Furthermore, SrTiO3 has been grown on silicon using thick oxide layers (60–120 Å) of SrO or TiO. See, e.g., B. K. Moon et al., Jpn. J. Appl. Phys., Vol. 33, p. 1472–1477 (1994). These thick buffer layers, however, would limit the application for transistors.
Accordingly, a need exists for a semiconductor structure that provides a high quality monocrystalline film or layer over another monocrystalline material and for a process for making such a structure.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.